Optical amplifiers are key components in the design of many planned fiber optic communication systems. There are many advantages to an optical amplifier that also produces an electrical signal proportional to the optical signal passing through, in effect acting both as an amplifier and as an optical tap. The ability to simultaneously amplify and detect an optical signal has lead to a number of applications for semiconductor optical amplifiers including lossless signal tapping and photonic packet switching. Conventional approaches monitor the terminal voltage applied to a semiconductor optical amplifier as described by Koayashi and Kimura in the Journal of Lightwave Technology, vol 1, pages 394-401 (1983) and by Alping, Bentland and Eng in Electronics Letters, vol 20, pages 794-795 (1984). A hybrid combination of an optical amplifier, a fiber splitter to divide the output signal, and a high speed photodetector to detect the signal in one branch of the splitter have also been utilized.
The first approach suffers from a bandwidth limitation due to the carrier lifetime of the forward-biased amplifier. While two-section (or split contact) semiconductor amplifiers offer greater photodetector responsitivity and additional flexibility for signal processing and/or signal modulation techniques than single contact semiconductor optical amplifiers, the electrical frequency response of all semiconductor optical amplifiers operating as detectors is limited by this carrier lifetime which varies from a few GHz in quantum well amplifiers to less than 1 GHz in bulk devices. This is a significant limitation in view of the increasingly high data rates required of lightwave systems. Furthermore, the signal intensity from the terminal voltage of the amplifier is reduced in comparison to that from a typical photodetector because of the poor impedance matching to a normal 50 ohm microwave environment.
Splitting the signal also has disadvantages. The use of hybrid components results in a device that is bulky and complex. If monolithic integration were attempted the loss due to the splitting of the signal in a monolithic waveguide is typically excessive.
It is thus desirable to fabricate a monolithically integrated optical amplifier having a photodetector tap which provides for only a small loss in signal power due to the tap.